1. Field of the Invention
The invention relates generally to a device for transmitting electronic signals and more specifically to a device for transmitting an electromagnetic signal having a flat spectrum that produces little interference with other communications signals utilizing.
2. Description of the Related Art
Chaos is a complex form of motion that is not periodic and never repeats itself produced by systems which contain both some form of instability (such as a positive feedback) and at least one nonlinearity. The chaotic system produces motions that are almost periodic, however, as exemplified by large spikes in the power spectrum, but it never actually repeats. What occurs is that there is some instability in a chaotic circuit caused by an unstable feedback that makes any sort of periodic motion unstable. If two chaotic circuits are started off with a small variation in initial conditions their motion will diverge exponentially; therefore, chaotic motion is unpredictable. The signal exiting a chaotic circuit will be a chaotic signal.
It is easy to produce complex chaotic signals using simple analog electronic circuits, so chaotic circuits can make very simple generators for broadband signals. A chaotic system is nonlinear and produces a broadband signal. The chaotic signals are not periodic and never repeat, but in some cases they may contain signals that are almost periodic.
There are many different methods for removing the periodic components from a chaotic signal. One may remove the periodic components directly with bandstop filters, or isolate the periodic components with bandpass filters and subtract from the chaotic signal, or reproduce the periodic components without filters and subtract from the chaotic signal.
The behavior of chaotic systems has been well studied in recent years. Because chaotic systems contain instabilities, they have broad power spectra, although there may also be some narrow features in the chaotic spectrum. If these narrow features are removed, only the broad spectrum remains. As previously stated, chaotic systems are nonlinear, however, so that the narrow parts of the spectrum still exist, but they are mixed with the broad parts. Applying a nonlinear function to the chaotic signal can restore the narrow parts of the signal. It is possible to encode information of the narrow part of the chaotic spectrum, remove the narrow part of the spectrum so only a broad-band signal is present, and then recover the narrow band part in a receiver in order to read the information.
It is well known that chaotic signals are broad band, nonperiodic signals and that they may be produced by simple electronic circuits. In addition, some chaotic systems produce signals that are cyclostationarity, which means that a signal, y(t), from the chaotic system can have a mean E[y(t)] which is nonstationary and is a periodic function of time, where E is the expectation of y(t), this is well known to those skilled in the art. One method for detecting cyclostationarity in a signal is to take the autocorrelation of the power spectrum. Using a well-known theorem that states that the cross-correlation of two signals is equal to the product of their Fourier transforms, the autocorrelation of the power spectrum of a signal is proportional to the square of that signal. Therefore, any function which includes taking a product of a chaotic time series with itself may be used to detect cyclostationarity in that chaotic time series.
For certain applications, such as garage door openers, remote controls, portable phones, etc., the Federal Communication Commission (FCC) has set aside frequency bands for commercial communications that are unlicensed. One requirement to use these bands is that the transmitter have a flat spectrum to avoid interfering with other communications systems. The regulations promulgated by the FCC are designed to prevent the unlicensed devices from interfering with other communications. The regulations require that the transmitted signal have a relatively flat power spectrum.
There are well known spread spectrum technologies consisting of frequency hopping or direct sequences techniques which produce transmitted spectra that fall within the within the FCC rules. Existing spread spectrum methods can meet these requirements however, a problem is that the receivers and transmitters comprising these systems are complicated and therefore expensive. These circuits need digital circuitry to generate pseudorandom numbers which have to be synchronized with the receiving end. This synchronization is usually accomplished through the use of a preamble on the signal.